1. Field of the Invention
The present invention relates to an electroluminescent element and manufacturing method thereof.
2. Background of the Invention
An organic electroluminescent element includes a laminated structure which has an organic compound layer between an anode and a cathode. When a forward bias voltage is applied between the anode and cathode, an electron and a positive hole recombines in the organic compound layer, and the organic compound layer emits light. An electroluminescent display panel in which an image is displayed by arranging a plurality of organic electroluminescent elements, each of which emits light of red, green, and blue, on a substrate in matrix has been realized.
In addition, in Japanese Laid-open Patent Specification No. 2000-353594, it is disclosed that among organic compound layers, there is a type that can be laminated on an electrode by an ink jet method, which is a wet coating method. In this method, an electrode is first made lyophilic by conducting oxygen plasma processing to the electrode, and when the electrode is subsequently coated with an organic compound containing solution, the organic compound containing solution coated on the electrode spreads over the anode entirely. Thus, an organic compound layer with a uniform film thickness can be formed.
In an electroluminescent display panel, organic electroluminescent elements are formed on a transparent substrate. There are mainly two types, a bottom emission type in which light emission is taken from the transparent substrate side, and a top emission type in which light emission is taken from the other side to the transparent substrate side. Concerning the bottom emission type, in case of an active matrix driving type, an opening ratio (ratio of luminescent area) of a sub pixel decreases due to the pixel transistor and the like that are provided on the substrate. However, concerning the top emission type, there is an advantage in that opening ratio can be improved without influence of pixel circuit and the like, since organic electroluminescent element can also be arranged on the pixel transistor.
Concerning the top emission type, it is required to form a pixel electrode and a monolayer or a plurality of layers of carrier transport layer including the organic luminescent layer, on a substrate, and form a counter electrode which serves as a transparent conducting layer thereon. As for a material used for the transparent conducting layer, metal oxides such as tin-doped indium oxide (ITO) and the like can be mentioned, and as a film forming method of such material, a film forming method such as sputtering and electron beam can be mentioned. However, with these film forming methods, the underlying carrier transport layer is damaged. Further, in a case where a metal layer with a low work function is provided between the metal oxide and the carrier transport layer in order to improve the injection efficiency of the carrier, a problem occurred in that the metal layer is oxidized by a trace amount of oxygen that exists in the processing.